NPL 1 introduces a method of installing a ridge filter in a beam delivery system. The ridge filter in NPL 1 is configured so that bisymmetric mountain-like structures are arrayed in one line in a lateral direction.
In addition, PTL 1 discloses the following structure in order to sufficiently spread out a Bragg peak width. The structure includes a ridge filter provided with a function to disperse a range of abeam. The structure configuring the ridge filter has a point symmetric shape and a bilaterally asymmetric shape in an iterative direction of the structure. Thicknesses in the iterative direction of an uppermost stream surface and a lowermost stream surface in a depth direction are equal to each other. A thick portion in the iterative direction of the uppermost stream surface and the lowermost stream surface is not present in the depth direction.